In our case, there has been a traditional observation of Labour Day as the start of the general election period. This means that we've never started the debates before the first week of September.
Also, there is a general reluctance on the part of the candidates to ever have a debate within 10 days to two weeks of the general election. There is a sense in some campaigns that debates freeze the campaign, and that the candidate has to stop and prepare and really focus on the debate.
Given all the other things that a campaign wants to get done during September and October, it has led us to believe that three presidential debates of 90 minutes apiece, without commercial interruption, is about right. In trying to go for four, there might be a problem to get candidate agreement, and I don't think two is enough to cover the number of topics that are key on both domestic and international policy.